Commit message: ecc: Add blinding for ECDSA.

* cipher/ecc-ecdsa.c (_gcry_ecc_ecdsa_sign): Blind secret D with
randomized nonce B.

--

Reported-by: Keegan Ryan <Keegan.Ryan@nccgroup.trust>
CVE-id: CVE-2018-0495
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
CVE: CVE-2018-0495
CWE: CWE-203
Vulnerability description: Libgcrypt before 1.7.10 and 1.8.x before 1.8.3 allows a memory-cache side-channel attack on ECDSA signatures that can be mitigated through the use of blinding during the signing process in the _gcry_ecc_ecdsa_sign function in cipher/ecc-ecdsa.c, aka the Return Of the Hidden Number Problem or ROHNP. To discover an ECDSA key, the attacker needs access to either the local machine or a different virtual machine on the same physical host.

Project: libgcrypt
File: ecc-ecdsa.c

_gcry_ecc_ecdsa_sign (gcry_mpi_t input, ECC_secret_key *skey,
                      gcry_mpi_t r, gcry_mpi_t s,
                      int flags, int hashalgo)
{
  gpg_err_code_t rc = 0;
  int extraloops = 0;
  gcry_mpi_t k, dr, sum, k_1, x;
  mpi_point_struct I;
  gcry_mpi_t hash;
  const void *abuf;
  unsigned int abits, qbits;
  mpi_ec_t ctx;

  if (DBG_CIPHER)
    log_mpidump ("ecdsa sign hash  ", input );

  qbits = mpi_get_nbits (skey->E.n);

  /* Convert the INPUT into an MPI if needed.  */
  rc = _gcry_dsa_normalize_hash (input, &hash, qbits);
  if (rc)
    return rc;

  k = NULL;
  dr = mpi_alloc (0);
  sum = mpi_alloc (0);
  k_1 = mpi_alloc (0);
  x = mpi_alloc (0);
  point_init (&I);

  ctx = _gcry_mpi_ec_p_internal_new (skey->E.model, skey->E.dialect, 0,
                                     skey->E.p, skey->E.a, skey->E.b);

  /* Two loops to avoid R or S are zero.  This is more of a joke than
     a real demand because the probability of them being zero is less
     than any hardware failure.  Some specs however require it.  */
  do
    {
      do
        {
          mpi_free (k);
          k = NULL;
          if ((flags & PUBKEY_FLAG_RFC6979) && hashalgo)
            {
              /* Use Pornin's method for deterministic DSA.  If this
                 flag is set, it is expected that HASH is an opaque
                 MPI with the to be signed hash.  That hash is also
                 used as h1 from 3.2.a.  */
              if (!mpi_is_opaque (input))
                {
                  rc = GPG_ERR_CONFLICT;
                  goto leave;
                }

              abuf = mpi_get_opaque (input, &abits);
              rc = _gcry_dsa_gen_rfc6979_k (&k, skey->E.n, skey->d,
                                            abuf, (abits+7)/8,
                                            hashalgo, extraloops);
              if (rc)
                goto leave;
              extraloops++;
            }
          else
            k = _gcry_dsa_gen_k (skey->E.n, GCRY_STRONG_RANDOM);

          _gcry_mpi_ec_mul_point (&I, k, &skey->E.G, ctx);
          if (_gcry_mpi_ec_get_affine (x, NULL, &I, ctx))
            {
              if (DBG_CIPHER)
                log_debug ("ecc sign: Failed to get affine coordinates\n");
              rc = GPG_ERR_BAD_SIGNATURE;
              goto leave;
            }
          mpi_mod (r, x, skey->E.n);  /* r = x mod n */
        }
      while (!mpi_cmp_ui (r, 0));

      mpi_mulm (dr, skey->d, r, skey->E.n); /* dr = d*r mod n  */
      mpi_addm (sum, hash, dr, skey->E.n);  /* sum = hash + (d*r) mod n  */
      mpi_invm (k_1, k, skey->E.n);         /* k_1 = k^(-1) mod n  */
      mpi_mulm (s, k_1, sum, skey->E.n);    /* s = k^(-1)*(hash+(d*r)) mod n */
    }
  while (!mpi_cmp_ui (s, 0));

  if (DBG_CIPHER)
    {
      log_mpidump ("ecdsa sign result r ", r);
      log_mpidump ("ecdsa sign result s ", s);
    }

 leave:
  _gcry_mpi_ec_free (ctx);
  point_free (&I);
  mpi_free (x);
  mpi_free (k_1);
  mpi_free (sum);
  mpi_free (dr);
  mpi_free (k);

  if (hash != input)
    mpi_free (hash);

  return rc;
}